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I see a lot of pictures with those "line" (arc) effects in the sky and I'm wondering how this technique is called (so I can google how it's done). :

picture with lines/arcs in it (Source: Luke Bhothipiti)

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2 Answers 2

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Those are usually called "star trails", and they come about as a result of leaving the camera's shutter open for an extended period of time. The center of the circle will be either the north or south pole, depending on which hemisphere you are in.

As often as not, though, the sky with star trails is captured separately from the foreground, and later swapped into the foreground scene. That would be cheating, except that light pollution often makes it impossible (in a practical sense) to capture the star trails and the foreground in the same exposure unless you are in a very remote area.

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  • \$\begingroup\$ thanks for the answer. can't believe that it's really done with an extremely long exposure. quite surprised. :) \$\endgroup\$
    – gsharp
    Jul 16, 2012 at 21:45
  • \$\begingroup\$ It's still cheating :-) (But, I know you know that). \$\endgroup\$ Jul 17, 2012 at 3:09
  • \$\begingroup\$ In A True Image from False Kiva, Wally Pacholka managed to create a similar effect in a very remote location with no light pollution. \$\endgroup\$
    – TRiG
    Oct 26, 2012 at 23:35
  • \$\begingroup\$ @TRiG a nice picture, but hardly a stellar example of star trails. \$\endgroup\$
    – ths
    Jan 4, 2018 at 23:07
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This is called Star Trails and you do it by creating a long-exposure photograph of stars as they appear to rotate about the poles.

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